As the advantages of fiber optic based communication and control of industrial processes becomes better known, increasing emphasis is being placed on various methods of simple, inexpensive, and reliable communication of low level optical power via fiber optics to the sensor site for making a desired measurement, and returning the measurement information on fiber optic paths to the control and measurement location. Among the many problems facing designers of such process control systems are the need for few, low light level optical paths and methods of accurately and reliably carrying out the measurements in such a way that the derived measurement information may be accurately communicated by means of fiber optic signals. In the application of resonant element sensors, it is especially important that low power, highly efficient sensors be developed to modulate the light available. One problem related to self-oscillating optical sensors is in achieving high opto-mechanical loop gain in order to reduce the optical power threshold needed to drive the resonator to reasonable levels.
Instruments are well known wherein the resonant frequency of a resonant element subjected to a force is a function of the tension (or compression) applied to that resonator. It has been recognized that a force measuring instrument can be based on this relationship by causing the resonator to vibrate while a tension or compression force is applied thereto and measuring the vibration frequency. An application of this principle for vibrating wire resonators is known from U.S. Pat. No. 4,329,775.
For the purpose of this limited description, "process control" includes both individual variable processes and complex multivariable processes involving a large number of controlled process conditions characterizable as physical parameters or "measurands", such as acceleration, fluid flow, flow rate, temperature, pressure, differential pressure, level, and the equivalents and derivatives thereof. "Resonant mechanical structure", "resonator", and "resonant element" as used herein generally refer to beam (hollow beam, cantilevered beam and cantilevered hollow beam, and double- or other multiple-beam elements), and ribbon, wire or other articles of manufacture, and their equivalents, all of which can be resonated at particular oscillation frequencies. Specifically included are tuning fork structures of the single- and double-ended varieties, as well as multiple tine tuning fork structures.
"Fiber optic", "optical fiber", and "optical power" path or pathway means and equivalent terms refer to single or multiple mode fiber optic communication paths.
As used herein, the term "optical power", light, or light flux includes electromagnetic radiation of wavelengths between 0.1 and 100 micrometers, and specifically includes infrared, ultraviolet, and visible electromagnetic radiated power. For simplicity, such electromagnetic radiated power may be referred to generally and without limitation as "light", optical flux, or optical power. Such optical power may also be described as "steady" or "continuous" or "unmodulated" in order to distinguish it from optical power signals which are modified to carry information. The term "optical power" specifically includes coherent and incoherent light power.
"Modulation" is used broadly herein, and it is intended to mean modifying (or the modification of) some characteristic or characteristics of a light beam so that it varies in step with the instantaneous value of another signal, and specifically may be used herein to describe amplitude modulation and frequency modulation. "Unmodulated optical power" refers to optical power which is unmodulated in this sense.
"Monochromatic" refers to optical power composed of a single wavelength. "Collimated light" refers to optical power having rays which are rendered substantially parallel to a certain line or direction.
"Fluid" includes gases and/or liquids. The term "force" is used to describe any physical parameter or phenomenon capable of moving a body or modifying its motion, and specifically includes force exerted per unit area (pressure) and any parameter or phenomenon capable of conversion to pressure.
"Photothermal effect" and "photokinetic effect", as used herein, refer to the phenomenon wherein photons striking a suitable surface or surface coating cause localized heating, such heating being sufficient to cause localized expansion of the coating or substrate, and thereby producing motion.